AGU journal highlights - 10 March 2004

03/10/04

The following highlights summarize research papers in Global Biogeochemical Cycles (GB), Geophysical Research Letters (GL), Journal of Geophysical Research--Space Physics (JA), Journal of Geophysical Research--Solid Earth (JB), and Radio Science (RS). The papers related to these Highlights are printed in the next paper issue of the journal following their electronic publication.

1. Assessing heavy metal contamination in soils

A new method to determine the background levels of heavy metals in soil may allow researchers to estimate the manmade contamination in a variety of soil and climate locations worldwide. Hamon et al. observed a strong association between soil iron and manganese and several environmentally important heavy metals in Australian and Southeast Asian soils. They then extrapolated the geochemical properties in various regions and used their method to estimate the natural quantities of potentially harmful metals, including arsenic, lead and zinc. Currently, background values for metals in soil are based on regional averages from sites assumed to have no manmade contamination. The traditional method does not, however, account for natural variability in elemental concentrations in the environment and may also be compromised in areas where there is a long history of human activity.

A long-term analysis of the tropical Atlantic shows the most complete record to date of the equatorial ocean's surface current variability. Arnault and Kestenare looked at sea surface topography data from the tropical Atlantic Ocean from 1992-2002 and constructed a detailed profile of the water's yearly and interannual variability, which can be used to estimate the local circulation and potentially infer global ocean patterns. The authors applied ocean altimetry records from the TOPEX/Poseidon satellite into existing circulation models, finding seasonal changes that largely agreed with previous studies based on climate data. However, most previous studies only looked at a single year or season and projected the causes of short- and long-term effects based on their limited data. The authors used the detailed information to search for links between small-scale abnormal events and large-scale climate events like El Nino and the North Atlantic Oscillation.

Title: Tropical Atlantic surface current variability from 10 years of TOPEX/Poseidon altimetry

A new method to evaluate atmospheric feedback can provide a faster and simpler technique to measure global climate sensitivity. Gregory et al. describe a procedure to contrast surface air temperature changes and atmospheric radiation flux to provide an estimate for the heat being applied to the climate system. The resulting measures can provide a useful prediction to assess the atmospheric response to external forcing events like increasing greenhouse gas or aerosol concentrations, or changes due to solar variability. The authors note that, unlike most existing methods to measure heat flux, the new modeling method does not require the global conditions to reach an equilibrium before they can evaluate the atmospheric changes. They conclude that their technique provides only an approximate measure of the surface change, but the simplicity of the method allows it to be easily applied to a wide variety of external forcings.

Title: A new method for diagnosing radiative forcing and climate sensitivity

Researchers have used a finely tuned imaging system to record the first direct observations of water in meteors entering Earth's atmosphere. Pellinen-Wannberg et al. adjusted two filters in the ALIS auroral imaging network during the 2002 Leonid meteor shower and present optical identification of water in the meteor tails as they entered the Earth's atmosphere. The authors tracked one of the meteors from the time it entered the upper atmosphere at high speed until it evaporated from frictional heating. The findings were unexpected, since it had been thought that meteoroids tended to lose their vapor content before entering Earth's atmosphere. The researchers set the auroral imaging system for specific elements, however, specifically sodium and combined iron/calcium atoms, the latter of which is indicative of meltwater that originated on the comet that initiated the Leonid meteors. They also confirmed that the Leonid meteors were relatively young, having passed by the Sun only a few times.

New observations indicate that current models significantly underestimate the sensitivity of ozone loss in the Arctic to changes in the stratospheric climate. Rex et al. analyzed ozone losses for 10 recent Arctic winters and show that an additional 15 Dobson unit (DU) thinning of the ozone layer occurs per degree Kelvin [per degree Celsius, per two degrees Fahrenheit] of cooling in the Arctic stratosphere--a value three times larger than estimates from current models. The authors also found that climate conditions in the Arctic stratosphere became more favorable to large ozone losses over the past four decades, with a tripling in the volume of air exposed to polar stratospheric cloud conditions during the coldest winters. Such clouds are known to initiate the transformation of chlorine to forms that are highly reactive to ozone. Because the clouds only form during cold winters, the researchers suggest that Arctic ozone losses of up to 100 Dobson units during the 1990s resulted from long-term cooling in the Arctic and increased halogen levels.

A new measure for estimating solar radiation from the ultraviolet "glow" of the upper atmosphere will allow scientists an additional tool for better understanding the connections between solar and terrestrial variability. Strickland et al. define a new physical quantity for measuring short wavelength solar radiation energy from extreme ultraviolet and some x-ray emissions that are responsible for producing a phenomenon known as far ultraviolet dayglow and maintaining the sunlit ionosphere. The measure can also provide an independent assessment of existing extreme ultraviolet energy models. The researchers used dayglow observations from the TIMED spacecraft over nearly a month in 2002 and found that their measure was closely tied with solar extreme ultraviolet energy as recorded by another spacecraft during the Sun's normal rotation and spikes from solar flares. The new findings contradict recent reports that solar ultraviolet radiation is underreported by a factor of four.

A new investigation that evaluates the biogeochemical impact from the flow of small rivers into the coastal ocean shows that the cumulative effect of many river plumes likely exceeds that of the largest river systems. Warrick and Fong show that flooding rates among smaller rivers, particularly tributaries from mountainous regions, cumulatively provide greater amounts of mass and momentum into the ocean than huge, well-studied rivers like the Mississippi and Amazon. The authors also provide information that can be used to mathematically model the impact from such small rivers and estimate their contribution into the world's oceans. They note, however, that sediment and biological material ejected from rivers varies widely, which can affect the coastal geochemical properties like nutrient dispersion and resulting algal blooms. The researchers suggest that the study can be used to track particulate materials from the hydrologic processes into the oceans.

Increasing concentrations of greenhouse gases are likely cooling the thermosphere and forcing its contraction, resulting in density decreases in this upper atmospheric region. Emmert et al. analyzed orbital decay rates of satellites and other objects that have flown continually for more than 30 years and found a decrease of 2-5 percent per decade in the thermosphere's density since 1966. Such thermospheric reductions reduce the drag applied to satellites and spacecraft by the thin atmosphere that causes a slow decay in space objects' orbit. The authors note that manmade greenhouse gases, particularly carbon dioxide, enhance atmospheric radiation at higher altitudes and promote a cooling effect in the thermosphere. Theoretical simulations of this effect suggest that thermospheric densities could by cut in half by the year 2100, thereby causing a 50 percent reduction in the orbital decay rates of both operational satellites and space debris.

Lower atmospheric winds are the most important factor in causing millisecond-length variations in the Earth's rotation rate on intraseasonal, seasonal, and annual time scales. Gross et al. analyzed 20 years of atmospheric and oceanic data and report that millisecond variations in the planet's rotation rate are also caused by upper atmospheric winds, changes in the distribution of mass within the atmosphere, ocean and surface pressure changes. The authors found that although the upper atmosphere contains only one percent of the atmosphere's mass, its winds can cause five to 10 percent of the observed seasonal length of day changes, because of their high speeds. They further note that variations initiated by upper atmospheric winds have a more significant effect on seasonal length-of-day changes than even the cumulative effects of oceanic current and ocean-bottom pressure variations. Seasonal, intraseasonal, annual, and decadal analyses show, however, that oceanic effects have a smaller impact on Earth's gravitational field and changing the length of day than zonal winds in the upper atmosphere.

Title: Atmospheric and oceanic excitation of length-of-day variations during 1980-2000

Solar maximum conditions can result in a three- to fivefold degradation in Global Positioning System accuracies compared to normal positioning values. Skone et al. analyzed millions of GPS records to determine the error rate on marine users from solar effects during extreme solar events. The authors measured the ionospheric effects that can cause GPS errors by estimating the exact location of fixed reference stations during the solar maximum conditions seen in mid-2000. Such estimates are needed for safety-critical maritime systems, such as navigation and surveying that rely on GPS. The researchers identified several anomalous areas where the GPS signals were dispersed by space weather effects and compared long-term data from several GPS reference networks to pinpoint the nature of the inaccuracy. The solar conditions provide the largest source of positioning error; other sources, such as atmospheric and weather changes can largely be mitigated through the correct selection and placement of antennae and using tropospheric models.